Detection devices with parabolic mirror are commonly used to search for the presence of particular objects in a determined geographical zone. The objects searched for may, for example, be metal parts or heat sources. The detection device is then stowed on board an aircraft or a satellite which overflies the geographical zone concerned.
The geographical zone may be traversed according to a scan corresponding to the displacement of the overflight machine. This displacement defines a band in the geographical zone, called a sweep, which possesses a ground width, along a direction perpendicular to the displacement of the machine, and determined by the characteristics of the detection device and by the altitude of the machine. The direction of displacement of the overflight machine is commonly called the azimuth, and the direction perpendicular to the displacement is commonly called the elevation, or else distance. A width of sweep which is all the larger makes it possible to reduce the number of passes of the machine above the geographical zone, that are necessary in order to fully cover the latter with mutually adjacent sweeps.
To increase the width of sweep, certain known detection devices make it possible to vary the elevation of the direction of detection between several determined values. Each direction of detection thus adopted corresponds to a secondary ground band, situated inside the sweep and called a sub-sweep. The width of the sweep then corresponds to the sum of the widths of the sub-sweeps. A greater sweep width may thus be obtained. By way of example, a sweep width of greater than 400 kilometers is achievable using 5 sub-sweeps, A mode of operation of an SAR device which allows scanning with several sub-sweeps is called “ScanSAR”, standing for “Scanning Synthesized Aperture Radars”.
Among the known detection devices which allow detection in several directions, may be mentioned the electronic scanning antennas, also called “Active Antennas” An active antenna is constituted by a matrix of detectors and a control system linked to each of the detectors. The control system combines elementary reception signals delivered by all the detectors, applying a phase shift associated with the position of the corresponding detector in the matrix to each elementary signal. The whole set of phase shifts used defines the direction of detection. An identical principle is used for a matrix of radar transmitters, to obtain a radiation emission focused in a determined direction. Devices with active antennas exhibit very good performance, but their complexity and their cost constitute significant drawbacks.
Other detection devices are known, which incorporate a parabolic mirror and a detector placed at the focus of this mirror. These devices are much simpler, and consequently less expensive than the previous ones. In a device with parabolic mirror, the direction of detection is determined by the orientation of the mirror. This orientation may be modified either by a movement of the mirror with respect to the machine on board which the detection device is stowed, or by a roll movement of the machine itself during its displacement. In both cases, the direction of detection varies relatively slowly. Therefore, such a device is not suitably adapted for carrying out a scan of a geographical zone with several sub-sweeps.
It is also known to place an active antenna of reduced dimensions in the focal plane of a parabolic mirror, at the level of the focus of the mirror. The operation and the characteristics of such hybrid detection device result from a combination of the respective manners of operation and characteristics of the active antenna and of the parabolic mirror. In particular, phase shifts are again introduced between several detectors/transmitters of the active antenna which are simultaneously activated to obtain a determined direction of detection. However, such hybrid devices are again complex and expensive.